The invention is directed to a single chip adaptive filter which utilizes updatable filter weights.
In many communication, control or terrestrial/cable television transmission systems, received input signals often consist of the sum of a desired signal and an undesired noise or interference. A signal processor is required to estimate or recover the desired signal in the presence of the additive interference noise. Significant research efforts have been dedicated toward this type of signal estimation problem. One conventional approach for solving these problems includes the use of an adaptive finite-impulse (FIR) filter which removes the interference and produces an output that approximates the desired signal. The filter weights required to achieve the optimum performance can be determined from a least mean square (LMS) algorithm based on a gradient optimization. Applications for real-time adaptive filtering techniques are in such diverse fields as adaptive control, ghost cancellation in terrestrial and cable TV transmission, channel equalization for communication and magnetic recording, estimation/prediction for speech processing, adaptive noise cancellation in electrocardiogram, etc.
The predominant hardware challenges for implementation of such filtering devices are the development of processors capable of providing a large number of computations with low power consumption and low cost production. The computations required are divided between those carried out for the realization of the FIR filter, e.g. requiring the repetitive calculation of a sum of products, and those calculations for the adaptation of the filter, e.g. requiring a large number of multiplication steps and simple IIR (Infinite Impulse Response) filters.
An important example of a specific application requiring massive computations at low cost is the problem related to ghost cancellation in television systems. Several attempts have been made for standardizing ghost cancellation systems, all of which are generally similar to one another as discussed in Tawil et al., "Field Testing of a Ghost Canceling System for NTSC Television Broadcasting", IEEE trans. on Broadcasting volume 36, no. 4, pages 255-261, 1990. A standard reference would be incorporated into the transmitted signal at predictable time intervals. The received signal, distorted by multipath transmission, would be passed through an adaptive FIR filter and at the predictable time intervals when the references known to be present, the weights of the filter would be adapted so that the actual output comes to closely resemble the standard reference. At the times when the reference is not present, the adaptation would stop, but the FIR filter would continue to filter the signal to suppress the multipath interference.
The ghost canceling application requires adaptive FIR filtering which utilizes at least a few hundred taps. Using conventional digital circuits, a ghost canceler would have to be a multi-chip system. For example, a block diagram of a ghost cancellation system 10 which is conventionally utilized for terrestrial and cable TV transmission is shown in FIG. 1. An analog base band video input signal from input signal source 11 is passed through an analog-to-digital converter 12 for conversion to a digital signal. The input signal is also provided to a synchronization circuit 13 for extracting synchronization signals and phase-locked sample clock signals. The digital video signal is then fed to a digital FIR filter 14 which utilizes electrically programmable filter coefficients. The digital video signal is also provided to a DSP processor 15 which examines a captured single line or "training signal" which is known to contain the reference signal. The DSP processor carries out an adaptation algorithm stored in ROM/RAM memory 16 in order to calculate the filter coefficients necessary to cancel any imperfections in the channel. The filter coefficients are then downloaded to the FIR filter 14, which in turn performs the filtering operation on the rest of the video signal. The output of the FIR filter is fed to a video rate D/A converter 17 for producing the output signal 18 which is supplied to the video signal receiver. This type of system inherently requires an expensive multi-chip configuration, however, due to the fact that ghost cancellation systems will ideally be provided in many television receivers, an inexpensive realization for carrying out the same filtering function is very desirable.